1. Field of the Invention
This invention relates to a filling composition for incandescent lamp, and incandescent lamp containing the same and its use.
2. Description of the Prior Art
Generally, the elevation of filament temperature in incandescent lamps improves their luminous efficiency. Such an elevation however accelerates the vaporization of tungsten and the vaporized tungsten attaches and blackens on the inside surface of glass bulb, thus reducing luminous efficiency and life expectancy.
To suppress the vaporization of tungsten, incandescent lamps enclose inert gas such as nitrogen gas and/or rare gas, for example, argon gas, krypton gas and xenon gas. It is known that, in this case, the higher the molecular weight of the enclosed inert or rate gas, the heat loss on glass bulb becomes much less, thus enabling an elevated filament temperature.
Also is known that xenon gas has the highest molecular weight among these inert or rare gases and exhibits excellent luminous characteristics and extended life expectancy when enclosed in incandescent lamps. Xenon gas however has the disadvantages that its low ionization potential is causative of arc discharge to accelerate the burnout of filament when excessively enclosed in glass bulb. Because of this, it has been recommended that when xenon gas is enclosed in an incandescent lamp to be operated at a relatively high voltage, other inert or rate gas such as nitrogen gas and krypton gas is used in combination to give a xenon gas content less than about 10% by volume against the whole enclosed gaseous ingredients.
For example, Japanese Patent Kokai No.2,068/79 and No.211,853/87 disclose that an incandescent lamp enclosing in the glass bulb a composition containing nitrogen gas, krypton gas and xenon gas to give a xenon gas content of about 2-10% by volume against the whole enclosed gaseous ingredients exhibit an elevated arc starting voltage and excellent luminous characteristics. Such an incandescent lamp is however low in xenon gas content and this hinders the xenon gas to sufficiently elicit its inherent desirable properties. Both Japanese Patent Kokai also disclose that when xenon gas is enclosed in an amount exceeding 10% by volume, arc starting voltage reduces to a level which is no longer acceptable to practical use.
Japanese Patent Kokai No.95,850/85 discloses an incandescent lamp which encloses a composition consisting of nitrogen gas and 80% by volume or more xenon gas. Although this incandescent lamp exhibits excellent luminous characteristics, it has the disadvantages that a reduced arc starting voltage limits its use for miniature incandescent lamps, operating voltage of 12V or lower, power consumption of 8W or less, as well as that it requires a special pitch for filament winding in order to give a reduced electric field intensity for arc suppression.
As described above, there has been available no incandescent lamp which encloses xenon gas to a level high enough to wholly elicit the desirably properties of xenon gas but hardly causes arc discharge when operated at a relatively high voltage, usually, at a voltage not lower than that of standard ac line, in particular, about 90-150V.